Optical moisture sensor and method of making the same

ABSTRACT

A soil moisture sensor uses a non-collimated light source and a photosensor, respectively, mounted at the foci of a transparent ellipsoidal plastic body. The dimensions of the body are such that emitted light rays are internally reflected toward the photosensor at the surface of the ellipsoid if the surface is dry, but refracted outwardly of the body when the surface is wet. The amount of light reflected onto the photosensor is thus a measure of the amount of moisture at the surface of the sensor. Direct illumination of the photosensor by the light source is prevented either by interposing opaque electronic components between them on a circuit board, or by taking advantage of light source characteristics to minimize the amount of transmitted light. If a circuit board is used, it is completely encapsulated against moisture penetration by fixing it in a carrier and molding the body around and onto the carrier to form a monolithic unit with the carrier and circuit board.

RELATED APPLICATIONS

The present invention is a continuation of U.S. patent application Ser.No. 11/214,101 filed Aug. 29, 2005 now U.S. Pat. No. 7,247,837, whichclaims benefit of U.S. Provisional Application Ser. No. 60/605,178,filed Aug. 27, 2004; both of which are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to optical moisture sensors for irrigationsystems, and more particularly to a soil moisture sensor using a solid,transparent ellipsoidal body with a non-collimated light source embeddedat one of its foci to reflect light toward a photosensor embedded at theother focus of the body if the soil contacting the surface of the bodyis dry, or to refract it outwardly of the body if the soil is wet.

BACKGROUND OF THE INVENTION

Optical sensors for determining the moisture content of the soil in anirrigation system are well known. They usually take the form of a prismor similar structure, in which a light beam projected into the prism isinternally reflected toward a photosensor such as a photodiode orphototransistor. (The term “light” in this application is meant toinclude infrared radiation). The amount of light received by thephotosensor depends on the amount of moisture present at the surfaces ofthe prism. This moisture changes the optical characteristics of theprism surface and thereby causes a portion of the beam to be refractedoutwardly of the prism, instead of being reflected inwardly toward thelight sensor. The amount of refraction, and thus the amount of lightreceived by the photosensor, translates into a measurement of thewetness of the soil.

It has previously been proposed in Benoit et al. U.S. Pat. No. 4,422,714to use a transparent half-ellipsoid body as a level sensor in acontainer of mineral oil. In that patent, a fiber-optic light guideconveying substantially collimated light from a light source to theellipsoid's surface is terminated at one of the foci of the ellipsoid,while a second light guide conveying light to a photosensor receivessimilarly collimated reflected light at the other focus of theellipsoid. If all or part of the convex surface of Benoit's body isimmersed in mineral oil, the resulting change in the index of refractionat the body-oil interface causes the light received by the photosensorto indicate not only the presence of a critical oil level but alsowhether it is rising or falling.

The above-described prior art construction is not, however, practicalfor soil moisture sensors because the presence of particulates in soilrequires using the maximum available surface area of the ellipsoidalbody as a reflection surface, so as to average the moisture effects overas large a surface of the sensor body as possible. This in turn requiresa wide-angle light source and a wide-angle photosensor at the foci ofthe ellipsoid. One solution to this problem is shown in my copendingapplication Ser. No. 11/214,100, filed on Aug. 29, 2005 and entitledOptical Moisture Sensor the contents of which are hereby incorporated byreference. That application discloses a cylindrical sensor with aninterior refracting surface that causes divergent light rays to berefracted into parallelism so as to make optimum use of the cylindricalsoil-contacting surface of the sensor.

A disadvantage of the sensor shown in the above-cited copendingapplication in cold and moist environments is the fact that an air spaceneeds to exist between the light source or photosensor and the internalrefracting surface. In a cold environment, condensation can occur inthat air space, and in a very moist environment, moisture can migratethrough the sensor material. In either event, these conditions mayadversely affect the parallelism of the internally refracted rays andmay require special manufacturing precautions.

The aforesaid disadvantage can be overcome by mounting a wide-anglelight source and photosensor in direct contact with a transparentellipsoidal body. This does, however, cause several other problems. Forone, a substantial portion of the light travels directly through thetransparent body from the light source to the photosensor without beingreflected by any body-air or body-water interface. Consequently, thesensitivity of such a sensor is substantially compromised.

Another problem arises in the manufacture of moisture sensors of thetype described due to the fact that the light source and photosensormust be maintained in exact alignment with the foci of the ellipsoidduring manufacture. This is necessary in order to produce consistentreadings among mass-produced sensors. Also, the difference incoefficients of expansion between the body material and the circuitboard on which the sensor's optical and electronic components aretypically mounted can cause minute cracks adjacent the board into whichmoisture can migrate. It is therefore necessary to so encapsulate thelight source, photosensor and associated electronics in the ellipsoidalbody that moisture cannot cause any discontinuities between them and thebody.

SUMMARY OF THE INVENTION

The invention solves the first problem mentioned above by mounting someof the non-light related circuit components (e.g. resistors, capacitorsand chips) of the moisture-sensing electronics on the circuit boardbetween the light source and photosensor so that they prevent anynon-reflected light from reaching the photosensor.

The invention solves the second problem by providing a plastic carrierthat firmly secures and aligns the circuit board with respect to themold in which the transparent ellipsoidal body of the sensor is formed,yet allows the body material to completely surround the board withoutany air interface in the light path, and to form a moisture-tight bondwith the carrier in the finished unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b are schematic vertical sections along the axis of theinventive ellipsoidal moisture sensor illustrating the opticalfunctioning of the sensor in dry and wet soils, respectively;

FIG. 1 c is a diagram illustrating the critical angles at the surface ofthe ellipsoidal body of the sensor of FIGS. 1 a and 1 b;

FIGS. 2 a-c are top plan, end elevation and side elevation views,respectively, of the sensor encapsulated with its carrier;

FIG. 3 is a perspective view of the finished sensor;

FIG. 4 is a schematic axial section of a typical spherical-nose IRED;

FIGS. 5 a and 5 b are polar and Cartesian representations, respectively,of the light energy distribution in an alternative embodiment of theinvention;

FIGS. 6 a-d are plan, side, end and schematic sectional views,respectively, of the alternative embodiment;

FIG. 7 is a perspective view of the alternative embodiment; and

FIG. 8 is an electrical diagram of a preferred circuitry for theinventive sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1 a and 1 b illustrate the functioning of the invention. Thesensor 10 consists of a circuit board 12 carrying a light source suchas, e.g., an infrared emitting diode (IRED) 14, a light sensing devicesuch as, e.g., a photosensor 16, and a component package 18. Thecomponents of the package 18 may, for example, include transformers,capacitors and/or resistors, or other components suitable for causingthe IRED 14 to produce appropriate illumination and to cause theillumination received by the photosensor 16 to be translated into usablesignals. In accordance with the invention, the package 18 is positionedon the circuit board 12 between the IRED 14 and the photosensor 16. Thepackage 18 is opaque and taller than the elevation of the IRED 14 andphotosensor 16 with respect to the circuit board 12, so as to shade thephotosensor 16 from direct illumination by the IRED 14.

An ellipsoidal body 24 of a transparent plastic such as cyclic olefincopolymer (COC) or acrylic polymer is formed over, and in intimatecontact with, the circuit board 12 and the IRED 14, photosensor 16 andpackage 18 positioned thereon. The IRED 14 and the photosensor 16 arewide-angle devices and are positioned, respectively, at the two foci 26and 28 of the ellipsoidal body 24. Therefore, any rays emitted by theIRED 14 between the limit rays 30 a and 30 n are reflected at theellipsoidal surface 32 of the body 24 toward the photosensor 16 as longas they impinge upon the surface 32 at an angle greater than thecritical angle C_(dry) (FIG. 1 c) if the surface 32 is dry, or thecritical angle C_(wet) if the surface 32 is wet. For acrylic as the bodymaterial, C_(dry) is 42.16°, and C_(wet) is 63.20°.

Thus, the dimensions of the ellipsoidal body 24 must be such that allrays 30 between the limit rays 30 a and 30 n impinge upon theellipsoidal surface 32 at an angle P between about 43° and 63°, which isgreater than C_(dry) but smaller than C_(wet) (the angle P is smallestfor the rays 30 a and 30 n, and largest half way between them). Anexamination of FIGS. 1 a and 1 b will show that the angle P in theexample described varies between 45° and about 52°. All rays 30 between30 a and 30 n are thus reflected toward the photosensor 16 when thesensor surface 32 is dry (FIG. 1 a), but are refracted outwardly of thebody 24 when the surface 32 is wet (FIG. 1 b). As the water content ofthe soil increases as a result of irrigation, more and more of thesurface becomes wetted and thus governed by C_(wet). Since P is lessthan C_(wet), the rays striking these portions of the surface will berefracted away, reducing the number of rays traveling to the photodetector. The signal from the photosensor is proportional to the amountof rays hitting it, so it becomes also proportional to the amount ofsurface that is not wetted. For most types of soil, the surface of thesensor will become wetted in a piecewise continuous manner with respectto the amount of moisture in the soil. Because the sensor 10 is normallyembedded in soil whose particulates attract moisture away from thesurface in some proportion to the lack of moisture content in the soil,the change from reflection to refraction is not sudden but gradual withincreasing moisture content of the soil. Consequently, the amount ofillumination received by the photosensor 16 is a measure of soilmoisture.

It will be understood that inasmuch as FIGS. 1 a and 1 b are axialvertical sections of the sensor 10, the rays 30 are actually half coneswhose tips are at the foci 26 and 28, and whose axes are parallel to theaxis A of the sensor 10. Consequently, the active or usable surface ofthe sensor 10 is the entire ellipsoidal surface 32 lying between thelimit rays 30 a and 30 n which form the minimum practical angle (about45°) with the surface 32. To facilitate manufacturing by injectionmolding, tapered cylindrical extensions 35 are provided on each end ofthe ellipsoidal portion. All rays other than those between rays 30 a and30 n are reflected or refracted away from the photosensor 16.

Because humidity can over time migrate through plastic into any air gapsthat may be in the light path, and because such humidity is likely toproduce light-scattering beads of condensate, it is important that therebe no air gap or air interface between the IRED 14 and the body 24 orbetween the photosensor 16 and the body 24. In order to prevent such anair gap, and in order to hold the IRED 14 and photosensor 16 in exactalignment with the body 24, the circuit board 12 of the inventive sensor10 is entirely encapsulated within the body 24 by injection molding oranother suitable manufacturing process. This is accomplished by tightlyfitting the circuit board 12 into a two-piece carrier 34 (best seen inFIG. 3) which, when inserted into the mold cavity of a molding machine(not shown), prevents any movement of the circuit board 12 during thepreferred injection molding process. The carrier 34 is equipped withspaced ribs 36 whose interstices allow the plastic material of the body24 to flow freely around it during the molding process. The ribs 36 alsoserve to hold the carrier 34 in the molding cavity so that it cannotmove during the molding operation.

In addition, care must be taken in the molding process to avoid theformation of bubbles in the area used by the light rays 30, and to makesure that the body material thoroughly “wets” the IRED 14 andphotosensor 16 without any air between them, for the same reason asdiscussed above.

The material of the carrier 34 is preferably of a type that bonds withthe material of the body 24 so as to form a tight seal with it duringthe molding of the body 24. The complete encapsulation of the circuitboard 12 and carrier 34 also prevents any migration of moisture into theelectronics if minute cracks form in the circuit board 12 due to thedifference in coefficients of expansion between the circuit boardmaterial and the material of the body 24.

The molding process incorporates the circuit board 12, body 24 andcarrier 34 into a monolithic sensor unit 10 shown in FIGS. 2 a-c and 3(the electrical wires interfacing the sensor 10 with external circuitryin an irrigation system are encapsulated with the circuit board 12 andare schematically shown as a cable 40 in FIGS. 2 a-c; they are not shownin FIG. 3). The completed unit 10 is then usable as is without furtherprocessing. It will be understood that the circuit board 12, its solderconnections and electronic components must be sufficientlyheat-resistant to withstand the high temperatures encountered ininjection molding.

An alternative embodiment 48 of the invention is illustrated in FIGS. 4through 8. In that embodiment, the half-ellipsoidal body 24 is replacedwith a full-ellipsoidal body 50. There is no circuit board 12, and theIRED light source 52 and photosensor 54 are mounted directly on acarrier 56 which is then encapsulated in the body 50 by injectionmolding or other appropriate process. The IRED light source 52 is of atype that has a current limiting resistor built into its housing, andboth the IRED 52 and the photosensor 54 are equipped with a sphericallens 58. IREDs and photosensors of that construction are standard itemsin the industry, and are widely commercially available.

The embodiment 48 has several advantages over the embodiment 10described above. First, the usable reflective surface of the sensor inembodiment 48 is about triple that of embodiment 10 for a given sensorsize, thereby making the sensor 48 much more accurate and reliable.Secondly, the absence of a circuit board eliminates the need for cautionin the molding process to avoid formation of moisture-attracting cracksin the circuit board 12 as discussed above, while at the same timereducing manufacturing costs. Also, the absence of a circuit board andthe incorporation of the current-limiting resistor in the IRED assemblyeliminates heat-sensitive solder joints. The IRED and photosensorassemblies have enough thermal mass to protect them against the briefthermal spike that occurs during the injection molding process.

Thirdly, as discussed in more detail below, the full encapsulation ofthe spherical lenses 58 dramatically reduces the direct, unreflectedtransmission of light from the IRED 52 to the photosensor 54, to thepoint where interposition of an opaque component between the IRED 52 andthe photosensor 54 becomes unnecessary. Fourthly, the use of a currentloop, discussed below, for conveying the output of the sensor to theelectronics which use its signal, improves the sensor's resistance tonoise and reduce its cost.

The essentially total elimination of direct light transfer from the IRED52 to the photosensor 54 without any intervening light barrier, inaccordance with the invention, takes advantage of the characteristicenergy distribution of spherical-lens IREDs. In this type of IRED, thelight source is a chip 60 (FIG. 4) that emits light mostly at about a 45degree angle to the axis 62 of housing 64. The glass lens 66 is shapedto focus this divergent light (as e.g. at 65) toward the axis 62 whenthe IRED 52 is in air.

Thus, in the intensity distribution diagram of FIG. 5 a, curve 60 showsthat the maximum light energy is emitted axially of the IRED assembly,and tapers off to zero in the direction transverse to the axis 62, whenthe IRED 52 is in air. When the IRED's glass lens is encapsulated,however, in a transparent material with a refractive index similar orequal to that of glass, such as COC or acrylic polymer, the focusingeffect of the lens 66 is nullified, and the intensity distribution ofthe IRED's light becomes substantially that of curve 68. Curve 68 showsthat in the encapsulation of the invention, the light intensity emittedby the IRED 52 thus peaks at about 45 degrees from the axis and isminimal in the axial and transverse directions.

As a practical matter, only light emitted at angles of about 10 degreesto 80 degrees from the axis 62 will usefully strike the surface of thebody 50 and be reflected (if the surface is dry) toward the photosensor54. Thus, the energy useful for moisture measurement in the fullellipsoid of embodiment 48 is that emitted between lines 70 a and 70 b,and between lines 72 a and 72 b, in FIG. 5 a. The total quantity ofuseful light is a function of the toroid whose axial cross section isthe area bounded by lines 70 a, 70 b and curve 68, and by lines 72 a, 72b and curve 68.

Light emitted by the IRED 52 in a cone of about 3 degrees on each sideof the axis 62, i.e. between lines 71 and 73, will strike thephotosensor 54 directly. Not only is that cone very small, but the lightenergy within that cone, as shown by curve 68, is minimal.Mathematically, because the plot of FIG. 5 a is in two-dimensional polarcoordinates, the angles need to be plotted along a Cartesian axis, andthe intensity needs to be multiplied by the sine of the angle toaccurately represent the three-dimensional reality of the situation.This is shown in FIG. 5 b.

As can be seen in FIG. 5 b, the ratio of direct light to reflectablelight is quite dramatic. By integrating the curve 68 between lines 71and 73, and dividing the result by the sum of the integrals of curve 68between lines 70 a and 70 b, and between lines 72 a and 72 b, the ratiocan be calculated to be approximately 0.0009. This minute error (lessthan 0.1%) caused by the direct illumination of the photosensor 54 isnegligible for all practical purposes.

FIGS. 6 a-d illustrate the ellipsoidal body 50 encapsulating the carrier56. Shown in those figures, but better visible in FIG. 7, are thetwo-wire connections that connect the IRED 52 and photosensor 54 inparallel. As shown in FIG. 8, the power supply 74 for the sensor 48 isequipped with a current sensor 76 which produces the output signal ofsensor 48. The current-limiting resistor 78 maintains the current drawnby the IRED 52 at a constant level, while the amount of current drawn bythe photosensor 54 varies in accordance with the amount of moisturepresent at the surface of the body 50. By connecting the IRED 52 andphotosensor 54 in parallel, a current loop is formed which requires justtwo wires 80, 82 in the sensor 48, instead of the conventional three orfour, for cost savings and easier manufacture. Also, it has been foundthat the current loop approach of FIG. 8 substantially reduces thesensitivity of the sensor 48 to extraneously induced electrical noise,which has been known to be a problem in irrigation installations.

It will be understood that the above-described embodiments are onlyrepresentative of the invention, and that its scope is to be limitedonly by the appended claims.

1. A method of sensing soil moisture comprising: providing a soil moisture sensor; placing at least an outer surface of a transparent region of said soil moisture sensor in soil; selectively transmitting a light within said moisture sensor; a portion of said light at least partially directed towards said transparent region; detecting a level of said portion of said light at least partially directed towards said transparent region; and determining a moisture level of said soil based on said level of said portion of said light; wherein said providing a soil moisture sensor further comprises minimizing condensation within said soil moisture sensor by providing a light source, a sensor and a circuit board that are encapsulated within said moisture sensor.
 2. The method of claim 1, wherein said selectively transmitting a light within said moisture sensor further comprises transmitting an infrared light.
 3. The method of claim 1, wherein said selectively transmitting a light within said moisture sensor is followed by reflecting at least a second portion of said portion of said light; wherein said at least second portion is proportional to said moisture level of said soil.
 4. The method of claim 1, wherein said selectively transmitting a light within said moisture sensor; a portion of said light at least partially directed towards said transparent region is followed by reflecting at least a second portion of said portion of said light against a curved region of said transparent region.
 5. The method of claim 4, wherein said curved region is half ellipsoid.
 6. The method of claim 1, wherein said minimizing condensation within said soil moisture sensor further comprises eliminating an air interface between a light source and a sensor within said moisture sensor.
 7. The method of claim 1, wherein said selectively transmitting a light within said moisture sensor further comprises transmitting a non-collimated light within said moisture sensor.
 8. The method of claim 1, further comprising blocking detection of non-reflected light within said moisture sensor.
 9. A method of determining soil moisture content comprising: positioning a soil moisture sensor within soil; said soil moisture sensor comprising a light source and a light sensor coupled to a circuit board; said soil moisture sensor, light sensor and said circuit board surrounded by said body member so as to eliminate an air interface; emitting a light within said soil moisture sensor; directing said light with a body member transparent to the outside of said soil moisture sensor; said body member having a first index of refraction directing said light to a light sensor, and said body and moisture in contact with said body having a second index of refraction directing said light away from said light sensor; measuring said light reflected from said body member with said light sensor; and calculating a soil moisture level based on a measured level of said light reflected from said body member.
 10. The method of claim 9, wherein emitting a light within said soil moisture sensor further comprises activating an infrared emitting diode.
 11. The method of claim 9, wherein said body member is comprised of a material selected from the following group: cyclic olefin copolymer and acrylic polymer.
 12. The method of claim 9, wherein said body member is shaped as a half ellipsoid or a full ellipsoid.
 13. The method of claim 9, further comprising blocking non-reflected light from said light sensor.
 14. A soil moisture sensor comprising: a transparent body, at least a portion of which having a curved shape; a light source positioned a first location within the soil moisture sensor; a light sensor positioned at a second location within said soil moisture sensor; wherein said light source is reflectable towards said light sensor when said transparent body is dry and reflectable away from said light sensor when said transparent body is wet and wherein said light source is not within direct view of said light sensor; and wherein said transparent body encapsulates at least said light source, said light sensor and a circuit board coupled to said light source and said light sensor; said transparent body preventing an air interface around said light source, said light sensor and said circuit board.
 15. The soil moisture sensor of claim 14, wherein said transparent body is half ellipsoid.
 16. The soil moisture sensor of claim 14, wherein said transparent body is ellipsoid.
 17. The soil moisture sensor of claim 14, wherein said soil moisture sensor is configured to minimize condensation within said soil moisture sensor.
 18. The method of claim 1, wherein said light source, said sensor and said circuit board are encapsulated by injection molding.
 19. The method of claim 9, wherein said light source and said light sensor coupled to said circuit board are encapsulated by injection molding.
 20. The method of claim 14, wherein said transparent body is formed by injection molding. 